A "straight shooter" is someone who is honest and forthright.straight shooter: Definition, Synonyms from Answers.com
I do not think that "straight shooter" can apply to your dialogue between Marcus and Jane. It's very awkward.
"Straight shooter" comes from the ability to shoot a gun straight and directly at a target.
Joe is a real straight shooter. He says exactly what is on his mind and is honest about his feelings.
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